Theory of Flight on How an Aircraft Can Fly

“Do they no see the birds above them flying wings spread out or folded? Nothing holds them aloft but God. All things are within His Purview” (Mulk 67:19).

“Do you not see the birds held high between the heaven and the earth? Nothing holds them (aloft) but God. There are verily signs in this for those who believe” (Nahl 16:79).

Theory of Flight :

Flight is a phenomenon that has been long a part of the natural world. Birds fly not only by flapping their wings, but by gliding with their wings outstretched for long distances. Smoke, which is composed of tiny particles, can rise thousands of feet into the air. Both these types of flight are possible because of the principles of physical science. Likewise, man-made aircraft rely on these principles to overcome the force of gravity and achieve flight.

Lighter-than-aircraft, such as the hot air balloon, work on a buoyancy principle. They float on air much like rafts float on water. The density of a raft is less than that of water, so it floats. Although the density of water is constant, the density of air decreases with altitude. The density of hot air inside a balloon is less than that of the air at sea level, so the balloon rises. It will continue to rise until the air outside of the balloon is of the same density as the air inside. Smoke particles rise on a plume of hot air being generated by a fire. When the air cools, the particles fall back to earth.

Heavier-than-air flight is made possible by a careful balance of four physical forces: lift, drag, weight, and thrust. For flight, an aircraft's lift must balance its weight, and its thrust must exceed its drag. A plane uses its wings for lift and its engines for thrust. Drag is reduced by a plane's smooth shape and its weight is controlled by the materials it is constructed of.

Understanding how air behave, many say how fluids flow, when we slice through the air at certain speed is incredibly important. Without the science of aerodynamics, as it’s well known, will not be able to design an aircraft. Thinking about how to move through a fluid, liquids and gases that can easily move or flow, quickly and effectively is really what aerodynamics talk about. If we want a more formal, scientific definition, we can say that aerodynamics is the science of how things move through air, or how air move around things.

Aerodynamics is part of a branch of physics called fluid dynamics, which is all about studying liquids and gases that are moving. Although it can involve very complex math, the basic principles are relatively easy-to-understand; they include how fluids flow in different ways,   what causes drag (fluids resistance), and how fluids conserve their volume and energy as they flow. Another important idea is that when an object moves through a stationary fluids, the science is pretty much the same as if the fluids move and the object were still. That's why it is possible to study the aerodynamic performance of an aircraft in a wind tunnel: blasting high-speed air around a still model of a plane is the same as flying or driving through the air at the same speed.

There are two types of how fluids flow: 1.Laminar flow or streamline flow, because the fluids flow in parallel lines, where things and the air sliding very smoothly pass one another in layers. 2.Turbulent flow, things and the air move in a more erratic way. If we're trying to design something, like an aircraft, ideally we want to shape the body so the flow of air around it is as smooth as possible, so it is laminar rather than turbulent. The more turbulence there is, the more air resistance the aircraft will experience, the more energy it will waste, and the slower it will go.

The speed in which fluids flow pass through an object varies according to how far fluids flow from the object. Right next to the object, the air speed is actually zero: the air sticks to the object. The further away from the object, the higher air speed flow. At certain distance from the object, the air will be traveling at its full speed. The region surrounding the object where the air speed increases from zero to its maximum is known as the boundary layer. We get laminar flow when the fluid can flow efficiently, gently and smoothly increasing in speed across the boundary layer; we get turbulent flow when this doesn't happen—when the fluid jumbles and mixes up chaotically instead of sliding pass itself in smooth layers.

Air resistance or drag, as it's usually known, follows on from the distinction between laminar and turbulent flow. In other words, drag is the force when the flow of air around it starts to become turbulent. Drag increases with speed. But a very important point is that drag doesn't increase linearly as speed increases but according to the square of speed, quadruple the drag.

It might seem obvious, but if fluid's flowing through or around an object, the amount of fluids at the end is the same as the amount at the start, called the continuity equation. More formally speaking, it says that the volume of fluid flowing in one place is the same as the volume flowing in another place. It follows from that the area through which the fluid flow multiplied by the velocity of the fluid is a constant: if fluid flow into a narrower space, it has to speed up; if fluid flow into a wider space, it has to slow down.

When fluids flow from one place to another, it has to conserve its energy. In other words, there has to be as much energy at the end as there was at the start. We know this from the fundamental law of physics called the conservation of energy, which explains that we can't create or destroy energy, only change it from one form into another. Think about the air flowing through the tube. The air just outside the tube, just where air blowing, has three types of energy: potential energy, kinetic energy, and energy because of its pressure. The air in the middle of the tube has the same three types of energy. However, because the air is moving faster there, its kinetic energy must be greater. Since we can't have created energy out of nothing, there must have been a reduction in one of the other two types of energy. As the air speed up, its pressure goes down. Since the air inside the tube is at a lower pressure than the air above it, the tube collapses until you stop blowing. Stated simply, Bernoulli's principle.

Bernoulli's principle can be derived from the principle of conservation of energy. This states that, in a steady flow, the sum of all forms of energy in a fluid along a streamline is the same at all points on that streamline. This requires that the sum of kinetic energy, potential energy and internal energy remains constant. Thus an increase in the speed of the fluid – implying an increase in its kinetic energy (dynamic pressure) – occurs with a simultaneous decrease in (the sum of) its potential energy (including the static pressure) and internal energy.

Bernoulli's principle can also be derived directly from Isaac Newton's Second Law of Motion. If a small volume of fluid is flowing horizontally from a region of high pressure to a region of low pressure, then there is more pressure behind than in front. This gives a net force on the volume, accelerating it along the streamline.

Fluid particles are subject only to pressure and their own weight. If a fluid is flowing horizontally and along a section of a streamline, where the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure; and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest.

In fluid dynamics, a vortex (plural vortices/vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in the wake of a boat, and the winds surrounding a tropical cyclone, tornado or dust-devil.

Vortices are a major component of turbulent flow. The distribution of velocity, vorticity (the curl of the flow velocity), as well as the concept of circulation are used to characterize vortices. In most vortices, the fluid flow velocity is greatest next to its axis and decreases in inverse proportion to the distance from the axis.

In the absence of external forces, viscous friction within the fluid tends to organize the flow into a collection of irrotational vortices, possibly superimposed to larger-scale flows, including larger-scale vortices. Once formed, vortices can move, stretch, twist, and interact in complex ways. A moving vortex carries with it some angular and linear momentum, energy, and mass.

 

Lift :

In order for an aircraft to rise into the air, a force must be created that equals or exceeds the force of gravity. This force is called lift. In heavier-than-air craft, lift is created by the flow of air over an airfoil. The shape of an airfoil causes air to flow faster on top than on bottom. The fast flowing air decreases the surrounding air pressure. Because the air pressure is greater below the airfoil than above, a resulting lift force is created. To further understand how an airfoil creates lift, it is necessary to use two important equations of physical science.

The pressure variations of flowing air is best represented by Bernoulli's equation. It was derived by Daniel Bernoulli, a Swiss mathematician, to explain the variation in pressure exerted by flowing streams of water.

Using the Bernoulli equation and the continuity equation, it can be shown how air flowing over an airfoil creates lift. Imagine air flowing over a stationary airfoil, such as an aircraft wing. Far ahead of the airfoil, the air travels at a uniform velocity. To flow past the airfoil, however, it must "split" in two, part of the flow traveling on top and part traveling on the bottom.

The shape of a typical airfoil is asymmetrical - its surface area is greater on the top than on the bottom. As the air flows over the airfoil, it is displaced more by the top surface than the bottom. According to the continuity law, this displacement, or loss of flow area, must lead to an increase in velocity. Consider an airfoil in a pipe with flowing water. Water will flow faster in a narrow section of the pipe. The large area of the top surface of the airfoil narrows the pipe more than the bottom surface does. Thus, water will flow faster on top than on bottom. The flow velocity is increased some by the bottom airfoil surface, but considerably less than the flow on top. The Bernoulli equation states that an increase in velocity leads to an decrease in pressure. Thus the higher the velocity of the flow, the lower the pressure. Air flowing over an airfoil will decrease in pressure. The pressure loss over the top surface is greater than that of the bottom surface. The result is a net pressure force in the upward (positive) direction. This pressure force is lift. There is no predetermined shape for a wing airfoil, it is designed based on the function of the aircraft it will be used for. To aid the design process, engineers use the lift coefficient to measure the amount of lift obtained from a particular airfoil shape. Lift is proportional to dynamic pressure and wing area.

Drag:

Every physical body that is propelled through the air will experience resistance to the air flow. This resistance is called drag. Drag is the result of a number of physical phenomena. Pressure drag is that which you feel when running on a windy day. The pressure of the wind in front of you is greater than the pressure of the wake behind you. Skin friction, or viscous drag, is that which swimmers may experience. The flow of water along a swimmer's body creates a frictional force that slows the swimmer down. A rough surface will induce more frictional drag than a smooth surface. To reduce viscous drag, swimmers attempt to make contact surfaces as smooth as possible by wearing swim caps and shaving their legs. Likewise, an aircraft's wing is designed to be smooth to reduce drag.

Like lift, drag is proportional to dynamic pressure and the area on which it acts. The drag coefficient, analogous to the lift coefficient, is a measure of the amount of dynamic pressure gets converted into drag. Unlike the lift coefficient however, engineers usually design the drag coefficient to be as low as possible. Low drag coefficients are desirable because an aircraft's efficiency increases as drag decreases.

Weight:

The weight of an aircraft is a limiting factor in aircraft design. A heavy plane, or a plane meant to carry heavy payloads, requires more lift than a light plane. It may also require more thrust to accelerate on the ground. On small aircraft the location of weight is also important. A small plane must be appropriately "balanced" for flight, for too much weight in the back or front can render the plane unstable. Weight can be calculated using a form of Newton's second law:

Thrust:

Propulsion involves a number of principles of physical science. Thermodynamics, aerodynamics, fluid mathematics, and physics all play a role. Thrust itself is a force than can best be described by Newton's second law.